In general terms, nitrile rubber is a copolymer of acrylonitrile and butadiene, wherein a bonding amount of acrylonitrile is 15˜50%, and acrylonitrile and butadiene are randomly copolymerized.
The primary advantage of nitrile rubber is excellent oil resistance due to polarity of a nitrile group in acrylonitrile. The property greatly depends on the amount of bonded nitrile. In a relation between the amount of the bonded nitrile and swelling, and in a relation between an aniline point of oil (lowest temperature at which hydrocarbon is completely dissolved in aniline having volume identical to the volume of hydrocarbon, ingredients of hard mineral oil such as gasoline may be confirmed by measuring an aniline point) and swelling, oil resistance is improved with increasing the amount of bonding nitrile and swelling is reduced with increasing an aniline point. Oil referred in the present invention means mineral oil having a petroleum-based hydrocarbon structure and is broadly used as a lubricant or a hydraulic oil of an oil pressure gauge.
Since such oils contain a variety of additives which may affect oil function, oil resistance of rubber at an aniline point is preferably assessed and the oils are used after measuring physical properties or swelling of oil used as a seal. In addition, when a compression permanent set test of an O ring was performed for long time in high-temperature oil, it was assumed that nitrile rubber had excellent characteristics, and nitrile rubber having volume changing in a certain range had better sealability, when compared to nitrile which showed good rubber properties but the volume of which was reduced. In this way, nitrile rubber has excellent oil resistance and thereby is ideally suited to use as an O ring, a V packing, and an oil seal to seal a lubricant, hydraulic fluid, a fuel oil and the like in fields such as industrial equipment, construction equipment, labor-saving machines, vehicles, aircrafts and the like, and, accordingly, is used the most abundantly. Although a temperature, at which nitrile rubber may be used, greatly depends on a combination condition, generally used temperature is −50 to −120 and, in particular, low-content nitrile rubber is used in machines or aircrafts for operation in cold environments.
In addition, relatively high-content nitrile rubber exhibits superior heat-resistance or mechanical properties and excellent gas permeability, and thereby may be used for vacuum down to approximately 10 torr. The relatively high-content nitrile rubber is polymerized through conventional low-temperature emulsion polymerization and is handled in a bale type after passing through a solidification process using a cohesive agent. However, requirements for cold-resistance of vehicle components are becoming more strictly and thereby demand for low-content nitrile rubber are increasing. However, the low-content nitrile rubber exhibits poor processability and a slow vulcanization rate. Therefore, there is a need to address the above problems.